FIG. 4 shows a block diagram of the last stage of a driver circuit with a complementary configuration in the conventional technology. The last stage of the driver is comprised of a bias circuit 40, output elements 31 and 32, and an output resistor 4 for output impedance matching. FIG. 5 shows a basic structure of a pin driver circuit corresponding to one test channel of an IC test system for driving a device pin of a semiconductor device under test (DUT). The example of FIG. 5 is a driver circuit without a temperature correction circuit.
The output elements 31 and 32 in this example are formed with CMOS transfer gates.
The output elements 31 and 32 in the last stage of the driver circuit change the amount of electric consumption according to the pulse waveforms and operating speeds. As a consequence, the junction temperature of the output elements also changes, which fluctuates the output characteristics of the driver circuit. Consequently, the output amplitudes and output timings vary from what originally intended.
FIG. 6 shows an example of drain current with respect to gate voltage in the output element when the temperature increases. In general, when the temperature in the MOSFET devices rises, the threshold voltage shown as Vt and drain current shown as Id will decrease. As a result, the drain current Id of the bias point 9 decreases from the original value. Because of this characteristics, there is a problem in the conventional driver circuit that the output level drops with an elapse of time as shown in an output signal 3 of FIG. 7(a).
However, when load resistance or the like is connected to the output or no load is connected, there is no deviation in the output level. Nevertheless, a switch timing and output impedance fluctuates.
The present invention is to maintain the output impedance of the driver circuit constant.
FIGS. 7(b)-7(d) show an example of timing deviations arise in an output signal 3. Namely, in the conventional technology, the intended delay timing 11 shown in FIG. 7(c) is changed to .DELTA.t as shown in FIG. 7(d). In this manner, the timing deviation is occurred by the temperature change in the driver circuit.
Further, for a driver circuit that requires a high degree of precision, an external apparatus must be installed to keep the temperature surrounding the driver circuit constant by a forceful cooler or an air conditioner, resulting in inconvenience in the application of the driver circuit.
As explained in the foregoing, in the driver circuit without a temperature compensation capability, the output amplitudes and output timings deviate from the originally intended ones because of the change in the heat dissipation accompanied by the change of power consumption in the output elements 31 and 32 in the driver circuit. Consequently, the driver circuit in the conventional technology is not able to provide the output amplitudes and timings of sufficient precision, causing problems in its actual use.